The invention concerns a multi-purpose subsea foundation element. Basically, the foundation element consists of a roof and a system of walls defining a number of cells. The roof is flat or slightly domed over each cell. The foundation element includes evacuation means for removal of the water entrapped in the cells and it is intended to be placed at the bottom of the sea with the walls completely penetrating down into the sea bed strata in such a manner that the roof of the element is essentially level with the mudline.
The primary applications of the foundation element are:
as an anchoring member in which application the element takes lateral mooring loads, for example from a floating structure/platform, or tension loads, for example from a tension leg platform; PA1 as pre-installed subsea bases in which application the element serves as a base element to which subsea installations or fixed above-water structures are mounted.
Prior-art installations for anchoring and subsea foundation purposes are primarily designed for either one or the other of the tasks outlined above. Anchoring devices designed to take lateral loads differ from devices designed to take tension loads. The disadvantages inherent in prior-art installations are considerable as will appear from the following.
Prior-art anchoring devices could be divided into those designed to take lateral loads and those designed to take tension loads. Conventional moored, semi-submersible platforms are representative of the former kind and tension leg platforms of the latter.
As regards lateral loads, prior-art anchoring devices designed for this purpose include self-penetrating marine anchors, single piles, piled anchor templates, gravity anchors and suction piles.
Conventional self-penetrating marine anchors have several shortcomings, especially as concerns safe performance and accessability for inspection. For instance, chains/wires and connections cannot be inspected. In addition, the as-installed position of the anchor in most cases is not determined. The lack of such vital information as the penetration depth of the anchor and the inclination of the anchor make prediction of anchor performance very difficult and uncertain.
When single piles are used in soft sea beds the chain/wire of the anchor cannot be connected to the upper end of the pile. Instead, it has to be connected at a level below the mudline. As a result, it is impossible to inspect the chains/wire and its connections.
Piled templates, being high-quality anchoring devices for lateral loads and allowing easy inspection of chain/wires and all connections, are very expensive.
Other types of lateral-anchorage devices are gravity anchors. These anchor structures are placed on the sea bed and the required anchoring function is provided through gravity. Gravity anchors provide high-efficiency anchorage but they are inherently heavy and thus expensive to handle. By using heavy ballast, applied after installation, their weight can be reduced.
Suction piles consist of short, large-diameter single piles which are installed in such a manner that they penetrate into the sea bed strata with the aid of suction. Anchors of this type are described for example in U.S. Pat. No. 3,469 900, GB PS Nos. 2 097 739 and PS 144 379.
The most important prior-art anchoring devices for tension loads are piled templates and gravity anchor structures. Single piles, including suction piles, are also used to some extent. The pros and cons of the various anchoring methods are essentially the same as those for corresponding installations for lateral loads discussed above. However, in single piles taking tension loads the connection point is situated at the top end of the pile, which makes it easy to inspect the connection members and the wires.
Subsea installations, such as drilling templates, are normally piled. This is especially the case in soft sea bed strata. In harder foundation soil shallow surface foundations are sometimes used. The use of piled templates is very expensive, particularly on deep-water sites.
An alternative solution for housing wellhead equipment is proposed in GB No. 2 133 060A. This patent specification teaches the installation of the wellhead equipment inside a subsea silo/caisson for protection. One or several suction piles mounted in a cluster, are positioned so as to penetrate into the subsea soil. The soil inside the suction pile is removed. Temporary and/or permanent floors and roofs are mounted inside the pile body and the wellhead equipment is housed entirely inside the structure thus formed.
As regards the foundation of fixed structures they can be categorized into pile foundations and gravity base foundations. Piling is by far the most common solution.
Piling of offshore structures is a well-proven and cost-effective technique for foundation of fixed structures. However, with increasing depths and platform sizes the piling-related costs tend to become excessive.
Gravity base structures which are pre-fabricated near-shore, are used extensively only in the North Sea. One reason for this geographic limitation is the lack of suitable deep-water near-shore sites in most other parts of the world. Various patented types of gravity base structures exist today. In gravity base structures, which are completely pre-fabricated before installation, the foundation is an integral part of the structure as such. Of special interest in this respect are the structures shown in NO B No. 135 909 (U.S. Pat. No. 3,961,489) and U.S. Pat. No. 3,911,697. These publications describe caisson-type gravity base structures which are equipped with very long skirts forming the foundation.
Other types of fixed above-water structures include articulated columns, guyed towers etc. The foundation of these structures could on the whole be subdivided into the same categories, i.e. piled foundations and gravity foundations. The guyed tower also include lateral load anchors for the mooring lines.